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Application Note 1159
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Power Good
PGOOD will be true (open drain) when the FB pin voltage is
within ±10% of the reference voltage and the soft-start
sequence is complete, i.e., once the soft-start capacitor is
finished charging. The status of PGOOD can be monitored
at the PGOOD test point (TP1).
Overcurrent Protection
The overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. Figure 7 shows the
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0 25 50 75 100 125
RT (k ? )
FIGURE 4. RT vs SWITCHING FREQUENCY
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overcurrent hiccup mode.
The overcurrent function protects the converter from a
shorted output by using the upper MOSFET’s R DS(ON) to
monitor the current. This method enhances the converter ’s
efficiency and reduces cost by eliminating a current sensing
Output Ripple
resistor.
Figure 5 shows the ripple voltage on the output of the
regulator at the free running 300kHz frequency and at
600kHz.
FIGURE 7. OVERCURRENT HICCUP MODE
Efficiency
FIGURE 5. OUTPUT RIPPLE
A resistor, R OCSET (R8), programs the overcurrent trip level.
The PHASE node voltage is compared to the voltage on the
OCSET pin, while the upper FET is on. A current (100μA
typically) is pulled from the OCSET pin to establish this
ISL6420 based regulators enable the design of highly
efficient systems. The efficiency of the evaluation board
using a 12V and a 5V input supply is shown in Figure 6.
voltage across an external resistor. If PHASE is lower than
OCSET, while the upper FET is on, then an overcurrent
condition is detected for that clock cycle. The pulse is
immediately terminated, and a counter is incremented. If an
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VIN=12V
VIN=5V
overcurrent condition is detected for 8 consecutive clock
cycles, and the circuit is not in soft-start, the ISL6420 enters
into hiccup mode. During hiccup, the external capacitor on
the ENSS pin is discharged and soft-start is initiated. During
soft-start, pulse termination limiting is enabled, but the
8-cycle hiccup counter is held in reset until soft-start is
completed.
The overcurrent function will trip at a peak inductor current
(I PEAK ) determined by,
I OCSET ? R OCSET
R DS ( ON )
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I PEAK = ---------------------------------------------------
LOAD (A)
FIGURE 6. EVALUATION BOARD EFFICIENCY
3
where I OCSET is the internal OCSET current source.
AN1159.0
November 10, 2004
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